The invention concerns a superconducting magnet system with a superconducting magnet coil system which is disposed in a cryogenic fluid tank of a cryostat, and a refrigerator provided for cooling the cryogenic fluid which cools the magnet.
A system of this type is disclosed e.g. in DE 100 33 410 C1.
Superconducting magnet systems generate high magnetic field strengths of the kind required e.g. in nuclear magnetic resonance (NMR) for both spectroscopic and imaging methods. The magnet coil configuration must be cooled in a cryostat in order to obtain a superconducting state for the magnet system. The cryostat comprises at least one cryogenic fluid tank in which a cryogenic fluid, e.g. liquid helium, is stored. The magnet coil system is also disposed in the cryogenic fluid tank. The cryogenic fluid thereby cools the magnet coil system. In its liquid state, the cryogenic fluid has a maximum temperature which corresponds to its boiling point.
Due to unavoidable heat input into the cryostat, the cryogenic fluid must normally be regularly refilled. This process causes downtimes and incurs expense, since the system is disturbed by refilling. For this reason, a refrigerator is implemented. The refrigerator has a cooling region past which the cryogenic fluid flows. Part of the cooling region is sufficiently cold to liquefy gaseous cryogenic fluid, which then drips into the cryogenic fluid tank.
DE 100 33 410 C1 discloses a cryostat configuration for receiving superconducting magnets, wherein a cryogenic fluid tank terminates in a neck tube of the cryostat, and a refrigerator is disposed in the neck tube. At least one separating body is provided which divides the neck tube into two partial volumes to guide the gaseous cryogenic fluid flow in a cycle.
When the refrigerator fails, a large amount of heat is input into the cryostat through cryogenic fluid convection flows and also through thermal radiation emitted by the refrigerator and/or the walls surrounding it. This heat input evaporates expensive cryogenic fluid. If an excessive amount of coolant is lost, the magnet must even be discharged.
Maintenance or repair of the refrigerator requires opening the refrigerator tower and entails the risk of quenching (i.e. a sudden breakdown of the superconductivity) the cooled magnet with an associated massive heat input into the liquid cryogenic fluid, causing large amounts of cryogenic fluid and/or very cold gas to suddenly escape. This could cause injuries to a maintenance technician or other individuals in the surroundings of the cryostat.
In contrast thereto, it is the object of the present invention to design the cryostat of a superconducting magnet system such that the heat input into the cryostat is reduced when the refrigerator fails, and the safety of the maintenance staff is improved in case of a quench.